1. Field of the Invention:
This invention relates to narrow band pass systems and more specifically to a digital filter that rejects all frequencies other than the desired pilot tone of a stereo AM radio receiver.
2. Prior Art:
Recent advances in the art of stereo AM radios, along with the decision to commerically market these radios, have emphasized the need to develop the most effective stereo AM receiver at the lowest price. A necessary component of a stereo AM receiver is a bandpass filter that determines when a left-right (L-R) signal is being adequately received, thereafter providing a signal that activates the stereo channel circuits of the receiver. When only the L+R signal is received, the mono channel alone is active.
Filters that convert two inputs of different frequencies into an output in an analog fashion are old in the art. Typically the output of a pair of bandpass filters, each tuned separately to one of the input frequencies, are compared and the greater output of the two bandpass filters determines the overall output. However, high Q analog filters may oscillate (ring) at their natural frequency creating a distortion in the passed signal.
Many types of digital filters are also old in the art. Such filters may be used in frequency-shift keying receivers which provide a logic "1" output for a first frequency input and a logic "0" output for a second frequency input. For example, U.S. Pat. No. Re.29,257 uses these binary outputs as input to a flip-flop which conditions an up-down counter using a clock independent of the flip-flop clock. See also U.S. Pat. No. 3,611,298, wherein bitts are represented by a group of substantially sinusoidal waveforms, and pulses having a given repetition rate are counted during each of the waveforms.
In general, digital filters of the prior art normally require external components, such as additional clocks, resistors, or capacitors and allow a wider range of frequencies to pass through the filters.
Thus, the need exists for an inexpensive narrow band digital filter, which does not ring, for a stereo AM radio receiver that requires no external components.